1. Field of the Invention
The present invention relates to a valve system used together with an automotive power steering system.
2. Description of the Prior Art
A conventional valve system for power steering contains a flow control valve which is adapted to open to by-pass most of the flow volume from a power steering pump to an inlet port via a by-pass port, as the speed of revolution of the pump increases in proportion to a vehicular speed, so that the controlled flow volume to the power steering system is kept nearly constant. But at high vehicular speed, it is desired that the amount of power steering assist which a vehicle driver receives be lessened so as to convey a safer sense to the driver. Such a system is disclosed in U.S. Pat. No. 3,314,495, wherein a by-pass system is provided with a spool valve which secures at one end a control rod having small and large diameter portions so as to reduce a flow volume to the power steering system by varying an effective cross-sectional area of a flow control orifice together with the control rod in response to the increase of the rate of revolution of the pump.
The prior valve system has an expedient reduced flow characteristic when a working load is not applied to the power steering system. However, when the power steering system is operated at high vehicular speed and working pressure is generated therein, the flow control valve is caused to move by the working pressure in a direction to close the by-pass port and thereby to widen the effective cross-sectional area of the flow control orifice so that the reduced flow volume is undesirably increased. Therefore, the amount of power steering assist increases and safety in driving at high vehicular speed is lessened.